A personal computer is typically coupled to peripherals that serve as computer-user interfaces. Such peripherals may include a keyboard, a mouse, and a video monitor. Typically standard cables connecting each peripheral are around four feet long. Because of the characteristics of the connecting cables and their interfaces, the cables do not provide a reliable peripheral connection when they are much longer than twenty feet.
There are situations where it is desirable to separate the computer from the peripherals at distances much greater than allowed by typical standard peripheral cables. The apparatus to extend the distance between a computer and peripherals is called a computer extender or a computer extension system. One such system, PC-Extender Plus is available from Cybex Computer Products Corporation of Huntsville, Alabama. The PC-Extender Plus allows a keyboard, monitor, and mouse to be placed up to 600 feet away from the computer. An extender typically has two electronic boxes and several cables between the computer and the peripherals. As the demand for extenders increases, the need for more efficient use of cables has developed.
In existing extenders, standard coaxial cables, shielded cables, and unshielded cables may serve as communication channels. In addition, custom cables have been developed to provide an improved communication channel for extender systems. The cables connecting the computer to the peripherals may be confined in a binder or may be separate cables for each of the desired communication channels. Because distances between the computer and the peripherals vary from a few feet up to several hundred feet the frequency response of the cable connecting a first site from a second site has variations. These variations are available from the cable manufacture or may be observed using well-known measurement techniques. The variation in the frequency response as a function of cable length is known to those skilled in the art. If the length of a cable is known, a filter with inverse frequency characteristics can be placed in series with the cable thereby effectively canceling the frequency response impairment of the cable. The use of this inverse filter is referred to as equalizing the characteristics of the communications channel and the filter is called an equalizer. If the inverse filter is tuned adaptively it is referred to as an adaptive equalizer. Telephone line modems, such as a V.34 modem, have adaptive equalizers. The adaptive equalizers in modems are implemented using digital signal processors and they work well at the relatively low-data rates of a telephone line modem.
Existing extension systems typically require a plurality of communication channels. Three may be used for the video signals having red, green, and blue video signals. Other cables or wires are required for transferring peripheral information such as mouse data, mouse clock, keyboard data, keyboard clock and other information. In existing extensions each of the cables may be equalized by a manual selection of filters. While this does improve end-to-end frequency response, the manual selection method may require custom designs for each cable length and may require the efforts of a skilled technician during setup. If the extension system is moved or for some other reason the cable lengths are changed it may be necessary to readjust the equalizing filters.
Accordingly, it is the object of this invention to provide for more efficient and improved method for equalizing cable characteristics and at the same time reduce cost. In addition if equalization were automatic it would not require the use of a skilled technician and would reduce the time to install or move a computer extension system.